Lignocellulosic biomass is viewed as an abundant renewable resource for fuels and chemicals due to the presence of sugars in the cell walls of plants. More than 50% of the organic carbon on the earth's surface is contained in plants. This lignocellulosic biomass is comprised of hemicelluloses, cellulose and smaller portions of lignin and protein. These structural components are comprised primarily of pentose and hexose sugars monomers. Cellulose is a polymer comprised mostly of condensation polymerized glucose and hemicellulose is a precursor to pentose sugars, mostly xylose. These sugars can easily be converted into fuels and valuable components, provided they can be liberated from the cell walls and polymers that contain them. However, plant cell walls have evolved considerable resistance to microbial, mechanical or chemical breakdown to yield component sugars. In order to overcome recalcitrance ground biomass is altered by a chemical process known as pretreatment. The aim of the pretreatment is to hydrolyze the hemicellulose, break down the protective lignin structure and disrupt the crystalline structure of cellulose. All of these steps enhance enzymatic accessibility to the cellulose during the subsequent hydrolysis (saccharification) step.
Pretreatment is viewed as one of the primary cost drivers in lignocellulosic ethanol and as a consequence a number of pretreatment approaches have been investigated on a wide variety of feedstocks types. The Saccharification of the cellulose enzymatically holds promise of greater yields of sugars under milder conditions and is therefore considered by many to be more economically attractive. The recalcitrance of the raw biomass to enzymatic hydrolysis necessitates a pretreatment to enhance the susceptibility of the cellulose to hydrolytic enzymes. A number of pretreatment methods, such as described in Nathan Mosier, Charles Wyman, Bruce Dale, Richard Elander, Y. Y. Lee, Mark Holtzapple, Michael Ladisch ‘Features of promising technologies for pretreatment of lignocellulosic biomass” Bioresource Technology 96 (2005) pp.673-686, have been developed to alter the structural and chemical composition of biomass to improve enzymatic conversion. A very recent comparison of “leading pretreatment” technologies was accomplished by the Biomass Refining Consortium for Applied Fundementals and Innovation (CAFI) and reported out in the journal Bioresource Technology in December of 2011. Such methods include treatment with dilute acid steam explosion described in U.S. Pat. No. 4,461,648, hydrothermal pretreatment without the addition of chemicals described in WO 2007/009463 A2, ammonia freeze explosion described in AFEX; Holtzapple, M. T., Jun, J., Ashok, G., Patibandla, S. L., Dale, B. E., 1991, The ammonia freeze explosion (AFEX) process—a practical lignocellulose pretreatment, Applied Biochemistry and Biotechnology 28/29, pp. 59-74, and organosolve extraction described in U.S. Pat. No. 4,409,032. Despite this, pretreatment has been cited as the most expensive process in biomass-to-fuels conversion (“Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production” Ind. Eng. Chem. Res., 2009, 48(8), 3713-3729.)
One pretreatment that has been extensively explored is a high temperature, dilute-sulfuric acid (H2SO4) process, which effectively hydrolyzes the hemicellulosic portion of the biomass to soluble sugars and exposes the cellulose so that enzymatic Saccharification is successful. The parameters which can be employed to control the conditions and effectiveness of the pretreatment are time, temperature, and acid loading. These are often combined in a mathematical equation termed the combined severity factor. In general, the higher the acid loading employed, the lower the temperature that can be employed; this comes at a cost of acid and its subsequent neutralization. Conversely, the lower the temperature, the longer the pretreatment process takes; this comes at the cost of volumetric productivity. It is desirable to lower the temperature because pentose sugars readily decompose to form furfurals and other species which represents a yield loss and these compounds are poisons to downstream fermentation. However the use of the higher concentrations of acid required to lower the pretreatment temperatures below that where furfural formation becomes facile (B. P. Lavarack, G. J. Griffin, D. Rodman “The acid hydrolysis of sugarcane bagasse hemicelluloses to product xylose, arabinose, glucose and other products.” Biomass and Bioenergy 23 (2002) pp. 367-380) requires sufficient quantities of acid that the recovery of the strong acid is an economic imperitive. If dilute acid streams and higher temperatures are employed the pretreatment reaction produces increased amounts of furfural and the acid passing downstream must be neutralized resulting in inorganic salts which complicates downstream processing and requires more expensive waste water treatment systems.
The amount of water employed in pretreatment further impacts the downstream energy balance and the overall economics of the fuel ethanol process. Further, there has been a recent review article studying the economic impact of total solids loading on enzymatic hydrolysis of pretreated corn stover produced by dilute sulfuric acid pretreatment (Humbird, D., Mohagheghi, A., Dowe, N., Schell D. J. “Economic Impact of Total Solids Loading on Enzymatic Hydrolysis of Dilute Acid Pretreated Corn Stover” Biotechnol. Prog., 2010, Vol. 26, No. 5, 1245-1251. (Published online May 26, 2010). It is thought that in a commercially relevant cellulosic ethanol process at scale, it will be necessary to carry out enzymatic cellulose hydrolysis on the whole pretreated slurry at a higher total solids loading. While it is mentioned in the article that it may be economically necessary, performing enzymatic hydrolysis at a high total solids loading remains challenging with reduced enzymatic yields. This is, in part due to an increase in the toxic impurities generated in the more concentrated pretreatment processes.